1. Field of the Invention
The present invention relates to linear beam devices having multi-stage depressed collectors, and more particularly, the invention relates to a multi-stage depressed collector having grooved surfaces in order to suppress generation of secondary electrons.
2. Description of Related Art
Linear beam electron devices are used in sophisticated communication and radar systems to convert direct current (DC) power into radio frequency (RF) power. Conventional klystrons, traveling wave tubes and inductive output tubes are examples of such linear beam electron devices. In a linear beam device, an electron. beam originating from an electron gun having a cathode is accelerated by a DC voltage differential with an anode spaced from the cathode. The accelerated electron beam passes through a drift tube containing an RF interaction structure. The electron beam may become amplitude modulated by applying an RF input signal to a grid disposed between the anode and cathode. Alternatively, the RF interaction structure of the drift tube may further include an RF circuit used to induce a modulation on the electron beam. Either way, the modulation results in electron concentration or bunching due to electrons that have had their velocity increased gradually overtaking those that have been slowed. The accelerated electrons of the electron beam give up varying amounts of their energy to the RF electric fields of traveling or standing wave circuits of the RF interaction structure. The energy removed from the electron beam in this manner may be subsequently removed from the device in the form of an amplified RF signal.
It has long been desirable to increase the efficiency of linear beam electron devices. If it were possible to make the length of the electron bunches infinitesimal and the amplitude infinite so that the average electron current remained finite, then one could apply an RF decelerating field to the bunch that would stop all the electrons and yield a device that is 100% efficient. In actual practice, when a sinusoidally time varying RF electric field exists on or in an output circuit of a linear beam device and the time length of the electron bunch is finite, some of the electrons will necessarily pass through the output circuit at times when the decelerating force of the RF electric field is less than maximum. As a result, many of the electrons will give up less than all of their energy, and the efficiency of the tube will be reduced accordingly.
A known technique for recovering the energy of the electrons that emerge from the output circuit (referred to as the xe2x80x9cspent beamxe2x80x9d or xe2x80x9cspent electronsxe2x80x9d) and thereby increase the efficiency of a linear beam device is to use a multi-stage depressed collector. A multi-stage depressed collector includes plural collector electrodes having successively decreasing voltage potentials in order to define a steady (i.e., not time varying) decelerating electric field. The collector electrodes further include holes aligned with the electron beam axis providing a path for the spent electrons to penetrate into the collector. The decelerating electric field slows the spent electrons as they penetrate into the collector to thereby allow their collection on one of the collector electrodes. The movement of the spent electrons within the collector is analogous to the way balls of varying velocity might roll up a hill, then stop and reverse direction after converting all of their kinetic energy to potential energy. If an electron has a little momentum transverse to the electric field when they reverse direction, the electron is likely to be collected by one of the electrodes that has less than the maximum potential and some of the energy of the spent beam will therefore be recovered. Unlike balls, electrons exhibit mutual repulsion due to their similar charge (i.e., negative) to thereby provide the transverse momentum.
Multi-stage depressed collectors are generally constructed such that most of the spent electrons will strike the back side of each of the collector electrodes (i.e., the side facing away from the output circuit), with the exception of the final collector electrode. This is advantageous since it tends to minimize the adverse effects of secondary electron emissions from the electrodes. A secondary electron emission refers to electrons that are knocked out the metal material of the collector electrodes by the impact of an energetic electron. These secondary electrons can actually become accelerated by the electric fields in the collector in a direction opposite the flow of the electron beam back into the linear beam device. By configuring the collector such that electrons typically strike the back side of a collector electrode, the electric fields operative on any secondary electrons that are emitted generally cause the secondary electrons to simply return to the electrode.
The shape of the final collector electrode remains problematic in terms of its generation of secondary emissions. Because an electron can only give up kinetic energy to the component of the electric field that is parallel to its direction of motion, it is desirable to configure the surface of the final collector electrode to be normal to the incoming electron trajectories. This shape also tends to cause secondary electrons to be accelerated back to higher potential electrodes and thereby waste power that is dissipated when the secondary electrons strike the higher potential electrodes. It is also known to configure the final collector electrode as a deep xe2x80x9cbucket,xe2x80x9d sometimes having a spike extending along the beam axis to shape the electric fields at the back of the collector to disperse high-energy electrons. A drawback of this design approach is that equipotential electric field lines at the mouth of the bucket are rarely perpendicular to the electron trajectories. Electrons that strike the surface of the bucket or the spike will usually have a great deal of energy in momentum that is directed parallel to these surfaces that cannot be recovered.
Accordingly, it would be desirable to provide a multi-stage depressed collector for a linear beam device having an electrode shape that minimizes secondary emissions while otherwise promoting efficient electron collection.
The present invention is directed to a multi-stage depressed collector for use in a linear beam device having a plurality of grooves formed in the collecting surface of at least one of the collector electrodes. The grooves provide a substantially field-free region that tends to prevent any secondary electrons generated by electrons that impact the grooves from exiting the grooves. Moreover, the grooves distort the electric field lines closely adjacent to the electrode surfaces to direct electrons into the grooves. As a result, a substantial reduction of secondary emissions are expected with the multistage depressed collector of the present invention, thereby providing a corresponding improvement in collector efficiency.
More particularly, a linear beam device comprises a cathode and an anode spaced therefrom, with the anode and cathode being operable to form and accelerate an electron beam. An RF interaction region having a drift tube is arranged relative to the anode to permit the electron beam to pass therethrough. A multi-stage depressed collector of the linear beam device has a plurality of collector electrodes successively arranged to collect spent electrons of the electron beam after passing through the RF interaction region. Each one of the plurality of collector electrodes has a distinct voltage level applied thereto defining a decelerating electric field within the collector. At least one of the plurality of collector electrodes further comprises a collecting surface having a shape that is normal to a coincident trajectory of the spent electrons, whereby a substantial portion of the collecting surface is covered with a plurality of narrow grooves.
In an embodiment of the invention, the grooved collector electrode further comprises the final electrode of the collector. The final electrode has a surface that is substantially spherical, and the plurality of grooves may be arranged in a concentric pattern of circles on the electrode surface. The plurality of grooves may be formed to a depth that is approximately twice a corresponding width. A region adjacent to an opening of each of the plurality of grooves comprises electric fields defining a convergent lens, thereby focusing the spent electrons into the plurality of grooves.
In another embodiment of the invention, the grooved collector electrode further comprises an intermediate electrode other than the final electrode of the collector. The plurality of grooves are disposed on a front side of the intermediate electrode oriented toward the cathode. The plurality of grooves are arranged in a radial pattern by which the grooves are closely spaced at a region of the collector surface adjacent to the central beam hole. Since relatively few of the electrons strike the front side of the intermediate electrode, and the electrons that do strike the front side tend to impact close to the central beam hole, the radial arrangement of grooves will substantially reduce secondary emission even though a large percentage of the overall surface of the electrode is not covered by grooves.